It is known by U.S. Pat. Nos. 4,208,386 and 4,325,924 to treat flue gases with urea [CO(NH.sub.2).sub.2 ] in the presence of oxygen to reduce noxious NO.sub.x (x=1 or 2) to elemental N.sub.2. The equations are elementary and need not be here discussed in detail other than to note that by-products of the treatment include water and carbon dioxide. Under ideal stoichiometric conditions one mole of urea reacts with two moles of NO.sub.x, but the actual molar flow will vary for many reasons explained below. The variation may be as much as 0.5 to 2 moles of urea per mole of NO.sub.x.
The treatment with urea helps to prevent so-called acid rain in that if NO.sub.2 is released to the atmosphere it can combine with atmospheric moisture to form nitric acid, HNO.sub.3. The treatment also helps avoid photochemical smog due to release of NO.sub.x to the atmosphere.
It is customary to dilute the urea with water, achieving a useful concentration in a solution which can be sprayed (atomized) into a furnace combustion column at a suitable point to reduce NO.sub.x to nitrogen. However, urea in solution imparts a high surface tension to the water. Consequently, in the preferred form of treatment, a surfactant is added to the urea solution. The surfactant reduces surface tension and therefore aids atomization (finer droplets) when the solution is sprayed into the stream of combustion gases before they attain the stack. The surfactant enlarges the spray pattern, promoting penetration and resulting in more effective contact with the combustion atmosphere.
Transportation of water is expensive and consequently the usual practice is for the manufacturer of the urea treating agent (solution) to ship a concentrated solution with a lower water content than actually required. The user, the operator of the combustion plant or facility, will therefore usually add dilution water. Dilution may also be done by a distributor. The amount of dilution will depend in part upon the estimate of the stoichiometric amount of urea needed for treatment of NO.sub.x in the flue gas. The stoichiometric amount of urea is the primary basis, but the amount of dilution will vary from plant to plant, and indeed can vary from day to day at any given plant. This is so because equilibrium of the chemical reactions involved is fuel dependent, temperature dependent and combustion rate dependent. The process is also dependent on oxygen concentration and the presence of reducing species such as carbon monoxide.
The necessary reaction to form N.sub.2 normally occurs within a narrow temperature band. The band is about 1700.degree.-2000.degree. F. Certain additives may lower the range. The reactions are rapid and less residence time is required for high temperatures than for low temperatures. If the temperature is too low ammonia (NH.sub.3) can result, known as "ammonia slip." Usually the best reaction takes place at about 1800.degree. F. where ammonia slip is slight. If the temperature is too high, NO.sub.x can actually be regenerated. To reduce these counterproductive possibilities, to minimize formation of ammonia and carbon monoxide, and to achieve other advantages evident from the discussion above, various chemicals in small amounts may be added along with the surfactant, especially chemicals which serve as scale inhibitors, keeping the equipment clean and preventing blockage of the spray nozzles.
Indeed, as will be explained, the boiler or furnace may be monitored to determine the heat pattern or profile to be expected for the stream of combustion gas so that the point of urea injection (flue gas temperature point) and urea concentration can be selected to assure superior reduction of NO.sub.x.
There are other chemical treatments besides urea for reducing NO.sub.x to N.sub.2 and the present invention could be applied thereto as well. In other words, there are other reactants which will react with NO.sub.x in the essential reduction process resulting in innocuous N.sub.2.
To use more reactant than needed can be wasteful if not counterproductive. On the other hand, when there is a drastic or sudden change in combustion parameters, the dilution may be too great, requiring a large dosage (increased concentration) of the treatment chemical used for reduction. The primary object of the present invention is, therefore, to enable a specified or needed concentration of a reactant in a water solution, for reducing NO.sub.x to an acceptable level, to be achieved at a plant where fuel when burned will release NO.sub.x to the stack. A related object of the invention is to enable the concentration of the needed reactant in the treating solution to be readily monitored and controlled to meet the required NO.sub.x reduction for specific performance of the furnace.
The supplier of a urea treating agent that will contain specialized additives will produce a concentrated solution for economy reasons mentioned. This concentrated solution will typically be concentrated far beyond the needs of the furnace operator and therefore will be diluted by the end user either prior to filling a holding tank or by direct in-line mixing as will be described below. The supplier or distributor or ultimate user can add a known amount of the tracer to the concentrated solution and then undertake a needed dilution, using a calibration standard under and in accordance with the present invention.